Peculiarities of optical absorption spectra of NdFe3(BO3)4 crystal in magnetically ordered state and at the transition to spiral magnetic phase

We study experimentally and theoretically optical absorption spectra of neodymium ferroborate in the temperature range of 6–32 K. We show that the temperature dependences of integral intensities of absorption lines demonstrate qualitatively different behavior for σ- and π-polarizations of the light. We suggest that this behavior may be caused by the appearance of a spiral magnetic structure in the compound under study. To describe the temperature dependences of lines intensities, we propose a theoretical model which takes into account the Dzyaloshinskii–Moriya interaction between Nd3+ and Fe3+ ions.